The Carb Shop Blow Through Holley - Blow Hard!

For Supercharged Performance You Need A Carb That Will Handle The Boost

Beautiful by design, awesome by performance, The Carb Shops' industry leading Blow Through Holley not only looks the part but also works to maximize fuel delivery under supercharged conditions.

Adding a blower for a substantial boost in horsepower and torque has never been more popular. Superchargers are all the rage among street enthusiasts who want big power while retaining the simplicity of a carburetor. Whether they choose a conventional Roots-style design or the easier-to-install centrifugal model, the increase in performance from either system is arguably the best bang-for-the-buck when all things are considered. By the same token, if not properly "dialed in," the term "bust" may be a more accurate description.

With a traditional Roots design, the carburetors are mounted on top of the blower, which sucks the air through their venturi and into the engine, allowing the oxygen to mix with the fuel that is drawn through the carb's boosters. While not the most efficient in terms of optimum fuel metering (which is why electronic fuel injection has become the best form of feeding most types of engines today), it is a time-proven system and can be tuned to work reasonably well for most applications. There are drawbacks, of course, which is why the popularity of the centrifugal design has grown by leaps and bounds.

But employing a carburetor to regulate the boosted air/fuel mix from a centrifugal blower has proven to be a little more complex than most people initially expect. The biggest reason is due to the fact that instead of sucking the charge through the carb, a centrifugal actually pressurizes the mix ahead of the carb's venturi and butterflies, which is referred to as a blow-through system.

By design, a carburetor works best when the engine's vacuum draws the air past the boosters and delivers an optimum-based on jetting, booster size, etc.-air/fuel mix into the intake system. Therefore, when air is forced into the venturi, in order to work correctly, the carburetor has to be modified to perform under what is essentially an opposite set of conditions.

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Standard fare for carb lovers, The Carb Shop starts with all the right parts from a Holley HP fuel meter. Before machining mods are made the carbs are completely cleaned and have all-new protective coatings applied.

That's where The Carb Shop has built its reputation as a leader in blow-through carb design. "We started doing testing on blow through carburetors in the mid-1970s when people said that you couldn't blow through a carb with more than four pounds of boost," explained Carb Shop owner Bob Vrbancic. "We took that as a challenge and developed our first style of blow-through carb on a turbocharged boat engine. Since then, we have made thousands of happy blow-through customers."

Working closely with the engineers at ATI ProCharger, and through ongoing trial and error testing on their state-of-the-art DTS dyno, Bob and his crew have come up with significant design modifications to the basic Holley four-barrel carb that will help anyone opting for a centrifugal blower attain the maximum performance level that the combination has to offer.

While a lot depends on such things as how much boost is created, camshaft overlap, compression ratio, and so on, the basic improvements that Bob has employed to his carbs gives the engine builder/car owner the best combination of drivability and maximum performance.

All Carb Shop carbs are custom-built to each application and are dyno tested, under boost, before being shipped to the customer. This is done to determine how the part-throttle drivablity is for the application the carb was built for. Then the transition circuit to wide-open throttle is determined for all out performance. According to Bob, just making a carb rich for a drag-race application won't work for on-street drivability.

Without giving up the store, recently we were able to pin Bob down as he described some of the meticulous tricks his technicians do to make his blow-through fuel meters the standard of the industry. Take a look as our camera literally zoomed in on some of those ever-so-minute modifications that have made blow-through applications so popular.

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Beautiful by design, awesome by performance, The Carb Shops' industry leading Blow Through Holley not only looks the part but also works to maximize fuel delivery under supercharged conditions.

Standard fare for carb lovers, The Carb Shop starts with all the right parts from a Holley HP fuel meter. Before machining mods are made the carbs are completely cleaned and have all-new protective coatings applied.

One of the main upgrades that The Carb Shop techs do in virtually all performance applications is to convert the carb to four-corner idling. Here is the hole drilled above the secondary butterflies in the baseplate that provides for better idle characteristics.

With boost being introduced through the carb, sealing is paramount. To accomplish this, the holes where the butterfly shafts protrude through the baseplate are given a small seal. First step is to machine them for the seal.

After a press-fit installation, the shafts have their ends radiused so they can be installed without pushing out the seal.

A little lube is used to help them through.

With the butterflies in place...

...another modification to help performance is performed when the primary to secondary slip linkage is relocated to allow the secondaries to open quicker.

Note the difference with the stock curved linkage (which results in the primary butterfly opening more before the secondary opens) and the straight link that gets the secondaries opening earlier, thus providing more fuel sooner.

One of the main upgrades that The Carb Shop techs do in virtually all performance applications is to convert the carb to four-corner idling. Here is the hole drilled above the secondary butterflies in the baseplate that provides for better idle characteristics.

One thing that isn't custom is the booster's installation. It fits snuggly in the stock location.

Here we see how the booster tube is filleted into the body.

This trick tool flares the end leaving a smooth transition for the fuel to flow from the blow-through the metering block and finally into the venturi.

With the boosters in place, each body takes its turn in the mill where the surfaces are machined flat and square, providing both a better seal and smooth and precise fuel metering. Usually the surface is cleaned up with one pass. But sometimes another cut is needed to smooth out the high and low spots.

The bottom of the main body also receives the squaring-up treatment. Once off the mill the sharp edges get deburred by hand.

Note how clean the machine surfacing is.

Another of The Carb Shops tricks; custom downfeed needle and seats are designed for more fuel delivery and reduced aerated fuel conditions.

The hidden secrets to The Carb Shop's success are contained in what starts out as a stock Holley metering block. Extensive custom modifications are made to the idle circuits, main wells and air bleed passages in an effort to maximize the fuel's path into the engine.

The metering block's main circuit receives these threaded brass inserts that have a specific emulsion hole drilled that corresponds to the needs of the customer's particular application. Note how large the main well transfer opening is.

Trick high-flow jet extensions insure the maximum amount of fuel transfer from the main jets. Note the modified oversize hole compared with the stock one.

ustom air bleed jets are also employed in The Carb Shop's custom blow-through fuel meters.

This maximizes the emulsion and idle circuits for better overall efficiency, drivability and most importantly, performance.

Last, but by no means least, is this custom crossover balance tube, which equalizes the float bowl chambers. In a boost-induced environment, too much pressure above the butterflies can create havoc with the fuel in the bowls. This can lead to irregular fuel flow through the boosters, or even no flow at all. This trick-of-the-trade is one reason that the blow-through fuel meters from The Carb Shop are making the power and performance they do.

Both blower designs, the centrifugal blower and roots blower, have merit and both have faults, so choosing the right blower is really a matter of defining your needs and realizing that neither is a cure-all power maker. - Super Chevy Magazine » Read More